[Freesurfer] MEG time series to surface overlay

Hi,

I have MEG time series for every vertex in a decimated surface (~6000 vertices). I want to create a file (i think is called overlay) in which the timeseries are saved, like an fMRI mgz overlay. How can I do that with matlab?

I have tried read_surf for reading the midthickness surface file, and then using the vertex coordinates and the MEG time series variable as input for write_wfile. however i get the error:

Error using bitshift Double inputs must have integer values in the range of ASSUMEDTYPE.

Error in fwrite3 (line 26) b1 = bitand(bitshift(val, -16), 255) ;

Error in write_wfile (line 58) fwrite3(fid, v(i)) ; % vertex number (0-based)

It seems that's not the function I need to use.

Thanks Dorothy